OT- Octal Fatness Amp Page Up

[Doug H]

I put up a page on the amp I just built:

http://home.cfl.rr.com/dbhammond/OF.html

It has a link to the schematic and the photo-essay I posted about yesterday.

Doug

[AL]

Thanks Doug. Are you going to post a layout?

AL

[Doug H]

Thanks Doug. Are you going to post a layout?

AL

Probably, eventually... I have one but I need to make some changes to reflect the final configuration.

Doug

[Paul Marossy]

I checked out the page last night, Doug. And I have the full schematic now. I'm already thinking about doing a chassis layout for it, but it will take me a little while to get to it. Any sound clips of it yet?

I think I might just build this one. I'm kind of stuck between doing this or a Matchless Lightning clone. I have a feeling that the Octal Fatness circuit might be the better sounding one of the two...

[Doug H]

I checked out the page last night, Doug. And I have the full schematic now. I'm already thinking about doing a chassis layout for it, but it will take me a little while to get to it. Any sound clips of it yet?

I think I might just build this one. I'm kind of stuck between doing this or a Matchless Lightning clone. I have a feeling that the Octal Fatness circuit might be the better sounding one of the two...

No sound clips yet, Paul. I will work on that next. I have a layout done but need to add a couple changes to reflect the as-built configuration. I will try to get that finished soon too and put it up at the site as well.

The Lightning is a Vox Top Boost kind of deal. FWIW, I am planning on doing my own spin on that amp sometime down the road, I have a few ideas. Don't know how this compares. The pentode compresses in a nice way that adds a lot of "airyness" and "chime" to the clean tone. It also adds what I would call "natural sustain" when it is cranked - big sustain without all the harmonics of a higher gain amp. This is not a high gain amp. I suspect it has more of an "American" or "blues" sound when cranked whereas the Lightning is probably more "British".  This is good headroom for me, a quarter turn on the gain control and my super-distortion is still pretty clean. The s/c neck pup stays clean much longer. Don't know how that compares with the voxy stuff.

One tweak I found that I liked was to simply put a 130p in parallel with the 360p treble cap in the tone stack. This lowers the ctr freq of the scoop down from 1KHz to around 600Hz or so. This allows more upper mids through and adds a lot of clarity when you are cranking it, esp to the neck pup. This gives it a real "marshall" character that sounds good for gainy tones. The downside is then it is not as "warm" sounding, esp for blues or cleaner tones. But both sounds are very useful so I may put that on a pull switch on one of the pots. It's amazing what a difference that one little cap makes.

One more thing, this amp is pretty loud, esp through a 1x12. (It ain't no Firefly.... ) I was playing with a keyboardist, bass player, and drummer and they asked me to turn down a little...

Doug

[Paul Marossy]

Hey Doug,

I actually meant to say Matchless Spitfire.  :oops:
Know anything about that one? I have heard good things about it, but I have never actually heard a soundclip of it... I think it might be a knock off a Marshall 18 watt, I'm not real sure.

Knowing your ears, I bet the Octal Fatness would be a good one. I'm not sure if it would be loud enough, though. At church we play between 90-100dB. I have to turn up my 60 watt Fender Hot Rod Blues DeVille about 3/4 of the way to max. volume (on the clean channel) to hear myself... I suppose the output section could use two tubes instead, but then it wouldn't be in an SE configuration anymore, would it. Although, on the other hand, wattage isn't necessarily a gauge of how loud it can be.

[Doug H]

Hey Doug,

I actually meant to say Matchless Spitfire.  :oops:
Know anything about that one? I have heard good things about it, but I have never actually heard a soundclip of it... I think it might be a knock off a Marshall 18 watt, I'm not real sure.

Knowing your ears, I bet the Octal Fatness would be a good one. I'm not sure if it would be loud enough, though. At church we play between 90-100dB. I have to turn up my 60 watt Fender Hot Rod Blues DeVille about 3/4 of the way to max. volume (on the clean channel) to hear myself... I suppose the output section could use two tubes instead, but then it wouldn't be in an SE configuration anymore, would it. Although, on the other hand, wattage isn't necessarily a gauge of how loud it can be.

I have never heard a Spitfire, so I can't help you there. I've heard they are supposed to be very good, though.

You could certainly do a push-pull output stage for the Fatness. That would give you more headroom as well as more output power. If you did that and wanted less-headroom/more-gain you could increase or eliminate R6. That is key to controlling headroom in this amplifier. I tried eliminating it and it really increased the gain.  It "fattened" things up quite a bit more too. In this case I left it in because I wanted more clean.

Doug

[Aharon]

"I actually meant to say Matchless Spitfire.  
Know anything about that one? I have heard good things about it, but I have never actually heard a soundclip of it... I think it might be a knock off a Marshall 18 watt, I'm not real sure."
Paul Marossy






I built a Spitfire.Although they share similarities they are diferent animals.
I like my SpitFire a lot,it can get a good SRV tone with a strat.
It's basically a clean amp that  breaks up when pushed to 10 similarly to let's say a very good Tube Screamer but it's the real thing.
Aharon

[Paul Marossy]

"I built a Spitfire.Although they share similarities they are diferent animals.
I like my SpitFire a lot,it can get a good SRV tone with a strat.
It's basically a clean amp that breaks up when pushed to 10 similarly to let's say a very good Tube Screamer but it's the real thing."

Different animal than a Marshall 18 watt? Sounds like a TS, but the real thing? Well, that wouldn't sound bad.

Thanks for giving me a heads up on that.

[Aharon]

Yeah,the 18Watt is a gainier amp but the topology, if you want t to call it that, is similarto the Spitfire.There's a few resistor value changes in the places that matter most like the PI etc.
Aharon

[Paul Marossy]

Well, I guess now I need to hear a soundclip of the Octal Fatness to make up my mind. A lot of people like the Marshall 18 watt. In fact, I think that there is a website devoted to them. I stumbled across it one day - 18watt.com or something like that.

Anyhow, I have a little chassis that's ready for a new circuit... plugging in the soldering iron right now... Ok, just kiddin' about the soldering iron.  :wink:

[Doug H]

Well, I guess now I need to hear a soundclip of the Octal Fatness to make up my mind. A lot of people like the Marshall 18 watt. In fact, I think that there is a website devoted to them. I stumbled across it one day - 18watt.com or something like that.

Anyhow, I have a little chassis that's ready for a new circuit... plugging in the soldering iron right now... Ok, just kiddin' about the soldering iron.  :wink:

Yeah, don't hold your breath...  It will be a few days before I get to any OF soundclips.

Doug

[Paul Marossy]

OK, I was turning blue there for a minute.  :wink:  :lol:

[Doug H]

Thanks Doug. Are you going to post a layout?

AL

I just added the layouts to my site:

http://home.cfl.rr.com/dbhammond/OF.html

Doug

[Eric H]

great looking layouts. I've never seen that mosfet circuit at the B+ center-tap --do tell.

-Eric

[Paul Marossy]

Darn it. I am running out of excuses not to build this thing.  :wink:
I too am wondering about that IRF 320 on the center tap...

[Doug H]

Darn it. I am running out of excuses not to build this thing.  :wink:
I too am wondering about that IRF 320 on the center tap...

Careful... That's "IRF820".

It is used to pull the B+ voltage down a little. The Electar transformer puts out about 450V, too hot for me. It's like using zener diodes. If you get a new transformer you don't need to worry with it. I would suggest a transformer that will give you  B+ around 350-400V. Something in the 250-0-250 to 275-0-275 range would probably be good.

Doug

[Paul Marossy]

Oh, 820.  :shock:

It was hard to tell on my printer...

So, then I could probably use a Hammond 269EX for this one, too?

[Doug H]

Oh, 820.  :shock:

It was hard to tell on my printer...

So, then I could probably use a Hammond 269EX for this one, too?

No, that's 190-0-190, probably too low. I'm sure Hammond has one that has a more appropriate voltage, can't remember the p/n:

http://www.hammondmfg.com/

Doug

[Paul Marossy]

Oh, duh.  :oops:
I was thinking that was 290-0-290, but as you pointed out, it's 190-0-190. I was thinking of the B+1 voltage...
Sorry, I went to bed too late last night.